1. Field of The Invention
The present invention relates to a frequency offset correcting circuit, and more specifically to such a circuit for use in a demodulator which forms part of an orthogonally multiplexed parallel data transmission system. This invention features an effective correction or removal of a dynamic or abrupt frequency offset caused by Doppler effect (for example).
2. Description of the Prior Art
It is known in the art that an orthogonally multiplexed parallel data transmission system allows spectrum overlappings within a predetermined bandwidth, and hence attains a very high efficiency of data transmission up to approximately the efficiency of the ideal Nyquist transmission. Therefore, such a transmission system has found demand wherein very high efficiencies of digital transmission are important.
In such a transmission system, parallel data are transmitted through a plurality of channels by modulating two carrier components 90.degree. apart in phase of each channel, while maintaining the orhogonality of adjacent channels. The relative relationship of carrier frequencies of all the channels can be kept constant during data transmission, but the carrier frequencies are liable to be shifted as a whole due to Doppler effect (for example). It is therefore necessary to especially correct this kind of abrupt frequency offset.
In order to correct the above-mentioned doppler shift, it is a current practice to provide a phase-locked loop (PLL) in a demodulator. More specifically, the phase-locked loop is arranged to correct the frequency offset using a pilot signal which is transmitted through a reference channel.
In accordance with a known demodulator having a doppler correction function, a second-order phase-locked loop is provided which includes a demodulating section. The demodulating section, however, includes a low-pass filter for channel separation or removal of interchannel interferences. This filter provides an incoming signal with a considerably large amount of delay time. Consequently, the loop gain should be lowered to ensure stable operation of the loop, and hence the known demodulator has encountered a problem that the phase-locked loop included therein is unable to follow abrupt frequency offsets caused by Doppler effect.